Do you know how BGP works? There are easily 50 different ways routing problems can cause outages like this. More than likely there was a compound failure which can cause all kinds of retarded behavior, including different networks getting different kinds of traffic, to say nothing of a plain old network service on a single net being down.
Routers can "crash" for different reasons, but atypically due to high traffic. If you really wanted to fuck with someone you make one BGP change. Only newbs use DDoS's. (Which, Anonymous being newbs, would be their MO, but unlikely they could DDoS a connectionless resource record database)
shrug I know how BGP works, yes. I think though from the symptoms of the issue it is going to end up being (If Godaddy is telling the truth about not being hacked), as you say, a compound failure. The exact cause of such a failure is left up to a truthful, full account of the outage being released. Further into this thread someone reported that an engineer from their side is going to release more information, so we'll see soon whom gets the prize :P.
Off the top of my head? One network they multihome had a weird packet loss only experienced by DNS and other services, so they tried to cut the routes over to the second network, but packets were still getting sent to the first network (which had DNS disabled but ICMP enabled on the hosts) and further router fuckage prevented them from switching back easily. Hell, they probably just couldn't get their BGP to propagate once they made the first change.
If you go with 'router tables' being the culprit, they probably had a core router that maxed out its RAM when they added another router in place, but they had already moved a part of the network that housed DNS by the time the routers synced and RAM filled from too many BGP lists to sort. You can still ping 'hosts' (which are I am almost certain a hardware load balancer and not an actual DNS host) while the DNS traffic is going nowhere because the backend DNS services were moved. Would take a couple hours to unfuck all of that.